While stopping short of an ultimatum, Luxembourg Prime Minister Juncker said after talks that Papademos can create the conditions for New Democracy leader Antonis Samaras to sign a letter required for Athens to get bailout funds.
Juncker said euro finance ministers set to meet in Brussels on November 29 "would like to be sure" that Papademos' commitments are "entirely shared by the main political leaders in Greece.
"I would think that from now until next Tuesday this will happen in an open way and then we'll be able to take a decision."
Juncker's "positive view," he said, was based on his "full confidence in the new Greek prime minister to create a situation where this cross-party agreement could be and would be delivered."
Like all the partners in the new unity government in Athens, Samaras is under EU and International Monetary Fund orders to commit to cuts, tax rises and other economic reforms -- otherwise, eight billion euros ($10.8 billion) in debt aid will remain blocked.
Juncker said he was "quite optimistic that we will be in a position from now to then to give a positive assessment" on releasing the monies.
Frozen since August, the loans are from last year's first Greek bailout and are essential to keeping default at bay in December, then paving the way for implementation of a new Greek bailout deal worth 230 billion euros agreed by eurozone leaders last month.
Juncker said he never expected Papademos to turn up with "a letter in his pocket."
However, he stressed: "It is up to the Greek government and to the leaders of the different political parties inside Greece to give us the possibility to check 'Yes' or 'No' if there is a cross-party agreement in Greece.
Should "there be no cross-party agreement, that disbursement of course could not take place," Juncker added.
Papademos did not address the issue directly during a news conference but told Greek reporters afterwards that there would be "no compromise" on the demand for Samaras to give his backing in writing to the commitments sought.